1. Field of the Invention
The present invention relates to a lighting system for operating a fluorescent lamp with dimming control, and also to a fluorescent lamp.
2. Description of the Related Art
Compact single-capped fluorescent lamps (hereinafter simply referred to as “fluorescent lamps”) are becoming prevalent for their high lamp efficiency as a light source of a lamp apparatus provided at commercial facilities and offices in a buried condition in a ceiling (hereinafter, such a lamp apparatus is referred to as a “downlight”).
There are various types of fluorescent lamps with different lamp powers for various locations and purposes of use. The fluorescent lamps are uniform in the outside diameter of a glass tube constituting a discharge tube and in a nominal lamp current. Each fluorescent lamp, however, differs from others in the lamp power due to a different length of the discharge tube and a different ratio by volume of neon and argon contents sealed within the discharge tube.
For example, when a fluorescent lamp is composed of a discharge tube having an inside diameter of 10.5 mm and a nominal lamp current of 0.32 A, the lamp power may be made to differ within the range of 16–57 W by varying the length of discharge tube within the range of 68–165 mm and the neon content by volume within the range 30–90%.
Further, when used with an electronic ballast, a fluorescent lamp may be “dimmed” so as to produce a smaller light output over a full range in response to reduction in the lamp current applied thereto. Such a dimmable lighting system is now in wider use.
Generally, electrode filaments of a fluorescent lamp are designed to reach an optimal temperature for thermoelectronic emission upon application of the maximum lamp current (approximately equal to the nominal value). In other words, when the lamp current is reduced for dimming, the temperature of electrode filaments is reduced. To compensate for the temperature reduction, a filament current is supplied to the electrode filaments additionally to the lamp current, so that the temperature of electrode filaments is maintained within an appropriate range.
Unfortunately, when operated at a dimmed level, above fluorescent lamps with high lamp-power (hereinafter, referred to as high-output fluorescent lamps) have following problems although such problems do not occur in fluorescent lamps with low lamp-power. That is, when operated at a dimmed level, ends of the discharge tube are blackened, and the electrode filaments are exhausted, which shortens life of the lamp.
These problems are caused in the following mechanism. In the case of a high-output fluorescent lamp, the neon content is high and thus the cathode voltage drop is large. In synergy with this, the temperature of electrode filaments rises excessively. Because of the excessively high temperature, the thermoelectronic emission material (hereinafter referred to as emitter) coated over the electrode filaments evaporates, and charged particles present around the electrodes are accelerated to cause sputtering of the electrode filaments to a greater extent. As a result, the electrode filaments are more quickly exhausted. In addition, when the lamp current is reduced for dimming, it equally means that the electronic current is reduced as well. As a result, electrons emitted from the surface of the electrode filaments are reduced, so that less cooling effect is achieved.